FIG. 1 shows a prior art implementation of a transceiving circuit for contactless communication. This transceiving circuit employs an integrated near field communication transmission module 2, type no. PN5xx, e.g. type no. PN511 or PN512 prior art matching is also valid for the Micore family (MF RC5xx, MF RC632, SL RC400), manufactured by NXP Semiconductors and external passive electronic components. The transmission module 2 is integrally equipped with transmitter means 3 being adapted to generate an electromagnetic carrier signal, to modulate the carrier signal according to transmitting data and to drive an antenna 5 with the modulated carrier signal, and with receiver means 4 being adapted to sense response signals being received at the antenna 5 and to demodulate the response signals. The transmission module 2 has output terminals TX1, TX2 being connectable to first and second transmitting paths wherein the transmitting paths are connected to the antenna 5, being represented in FIG. 1 by its equivalent circuit components capacitance Cant and inductance Lant. Between the output terminals TX1, TX2 of the transmission module 2 and the external antenna 5 the following devices are switched into the transmitting paths: an electromagnetic compatibility (EMC) filter comprising two inductors L0 and two capacitors C0; and an impedance matching network comprising ohmic resistors RQ and capacitors (not shown). It should be noted that during manufacturing of the transceiving circuits the antenna 5 is “tuned” by means of the impedance matching network.
Further, the receiver means 4 of the transmission module 2 comprise an input terminal RX that is connected to a receiving path that branches off from the first transmitting path. A phase adjusting capacitor C13 is switched into the receiving path in order to enable adjusting of the phase angle of signals between the first transmission path and the receiving path. By adjusting the phase angle an optimal demodulation can be achieved. Further, an ohmic resistor R1 is serially switched into the receiving path. With this resistor R1 the voltage level appearing at the input terminal RX of the receiver means 4 can be adjusted. Numeral VMID depicts an analog reference voltage input of the receiver means 4. A capacitor C14 is switched between the analog reference voltage input VMID and ground potential. An ohmic resistor R2 connects the input terminal RX and the analog reference voltage input VMID.
For a better understanding of the function of the RFID transmission module 2, a block diagram of the near field communication (NFC) transmission module type no. PN511 is shown in FIG. 2. The NFC transmission module 2 comprises analog circuitry which can be roughly divided into transmitter means 3 and receiver means 4. Although not shown, the analog circuitry comprises output drivers, an integrated demodulator, a bit decoder, a mode detector and an RF-level detector. A contactless UART communicates with the analog circuitry via a bus. The contactless UART comprises data processing means, CRC/Parity generation and checking means, frame generation and checking means, and bit coding and decoding means. The UART further communicates with a microprocessor, comprising a 80C51 core, ROM and RAM. A host interface enables to connect the transmission module to external devices. The host interface may comprise I2C, serial UART, SPI and/or USB interfaces. Further details of the transmission module can be looked up in the respective data sheets which are publicly available.
One of the most important field of application of near field communication (NFC) transmission modules are mobile phones. Mobile phones equipped with NFC transmission modules can be used for ticketing, access control systems, payment services, etc. Usually, the NFC transmission modules are powered by the hosting mobile phone. Nevertheless, specifically for ticketing applications, there is a strong demand that the NFC transmission module must still be operable when the battery of the mobile phone has been exhausted in order to keep the tickets managed by the NFC transmission modules available. This demand has resulted in considerations of using electric energy that is provided by an electromagnetic field generated by an external reading device. This so called “powered by the field mode” has already been implemented in standard NFC cards and standard cards where a powered by the field circuitry is directly connected with the antenna. This approach, however, has the inherent disadvantage that such a card cannot be operated in a so called “reader mode” where the NFC card plays the role of a reading device that initiates communication with target NFC devices. Operating an NFC device in the reader mode requires an external voltage supply since the NFC device itself has to generate an electromagnetic field. In order to enable the reader mode for such a card the powered by the field circuitry has to be physically removed because it interferes with the reader mode circuitry. This removing in turn renders the powered by the field mode impossible. Therefore, there is still a strong need for a transceiving circuit for contactless communication that allows the circuit to be operated in a reader mode, a powered by the field card mode, and a battery supplied card mode without requiring to physically add or remove any hardware parts.